Broadband laser ranging uses spectral interferometry and a dispersive Fourier transform to perform high repetition rate position measurements of explosively-driven surfaces typically moving at several km/s. A broadband fiber laser and fiber interferometer record distance as a relative delay between short pulses, and the beat spectrum of the pulses is mapped into the time domain via long propagation in dispersive fiber. Optical amplification and a fast oscilloscope allow the dispersed spectrum to be recorded in real-time, often at measurement rates of 20-40 MHz. The third-order phase of the dispersive fiber causes distortions in mapping the spectrum into time that must be compensated for when analyzing the measured data.
We characterize the accuracy and precision of BLR systems by performing a scan of static positions and comparing our single-shot measurements against position measurements from a commercial Michelson interferometer. We demonstrate a combination of hardware and data analysis that measures position to within 30 microns over a 27 cm range with very high precision.
Ultrashort pulses emerging from multimode optical fibers are spatiotemporally complex—the multiple fiber modes have different spatial shapes and different propagation velocities and dispersions inside fibers. To measure the complete spatiotemporal field from multimode fibers in real time, we propose and demonstrate a technique for the complete measurement of these pulses using a simple pulse characterization technique, called Spatially and Temporally Resolved Intensity and Phase Evaluation Device: Full Information from a Single Hologram (STRIPED FISH). It yields the complete electric field vs. space and time from multiple digital holograms, simultaneously recorded at different frequencies on a single camera frame.
Broadband laser ranging (BLR) is essentially a spectral interferometer used to infer distance to a moving target. The light source is a mode-locked fiber laser, and chromatic dispersion maps the spectral interference pattern into the time domain, yielding chirped beat signals at the detector. A BLR record is a sequence of these chirped signals, representing consecutive target positions. To infer distance to a target, each underlying pulse envelope must be consistently registered and subtracted despite environmentally-induced variability. Then, nonlinear transformation of the phase is applied to remove the chirp, an FFT is performed to determine the peak frequency of the de-chirped signal, and a calibration factor relating de-chirped frequency to distance results in target position. Here, these analysis steps are discussed in detail.
Broadband Laser Ranging (BLR) is a new diagnostic being developed in collaboration across multiple USA Dept. of Energy (DOE) facilities. Its purpose is to measure the precise position of surfaces and particle clouds moving at speeds of a few kilometers per second. The diagnostic uses spectral interferometry to encode distance into a modulation in the spectrum of pulses from a mode-locked fiber laser and uses a dispersive Fourier transformation to map the spectral modulation into time. This combination enables recording of range information in the time domain on a fast oscilloscope every 25-80 ns. Discussed here are some of the hardware design issues, system tradeoffs, calibration issues, and experimental results. BLR is being developed as an add-on to conventional Photonic Doppler Velocimetry (PDV) systems because PDV often yields incomplete information when lateral velocity components are present, or when there are drop-outs in the signal amplitude. In these cases, integration of the velocity from PDV can give incorrect displacement results. Experiments are now regularly fielded with over 100 channels of PDV, and BLR is being developed in a modular way to enable high channel counts of BLR and PDV recorded from the same probes pointed at the same target location. In this way instruments, will independently record surface velocity and distance information along the exact same path.
The general theory of first-order spatiotemporal distortions provides a very helpful framework for understanding beam couplings in ultrashort pulses. The theory describes both real and imaginary coupling terms between 4 pairs of dimensions. The imaginary coupling terms are difficult to understand and visualize because they are difficult to plot in a meaningful way. In general, plotting the spatiotemporal intensity and phase of pulses in in two and three dimensions is a difficult problem. Our work on pulse visualization provides an unprecedented opportunity to study spatiotemporal couplings in ultrashort pulses. We create movies of pulses as they would appear naturally, with all of their evolving spatial, temporal, and spectral structure readily apparent.
Multiple pulsing is a feature of most mode-locked ultrafast laser systems at very high pump powers, and slight variations in the pump power around certain regimes can cause sinusoidally-varying or even chaotic separations among pulses. The impact of this type of unstable multipulsing on modern pulse measurement methods has not been studied. We have performed calculations and simulations and find that allowing only the relative phase of a satellite pulse to vary causes the satellite to wash out of the SPIDER measurement completely. Although techniques like FROG and autocorrelation cannot accurately determine the precise properties of satellite pulses, they do succeed in seeing them.
We solve the problem of single-shot complete temporal measurement of continuum using cross-correlation frequency-resolved optical gating, achieving the necessary large spectral range using a polarization-gating geometry and the necessary large temporal range by significantly tilting the reference pulse. In addition, we simultaneously cancel the previously unavoidable longitudinal geometrical temporal smearing by using a carefully chosen combination of pulse tilt and beam-crossing angle, thus simultaneously achieving the required temporal resolution. The result is that we are able to make a complete measurement of an individual complex continuum pulse generated in photonic-crystal fiber. By enabling measurement of single optical rogue waves, this technique could provide insight and perhaps even lead to the prediction of when mathematically similar, destructive, oceanic rogue waves may occur.
We demonstrate ultrashort pulse spatiotemporal field measurement for multimode optical fibers, using a singleframe characterization technique, called Spatially and Temporally Resolved Intensity and Phase Evaluation Device: Full Information from a Single Hologram (STRIPED FISH). We measure STRIPED FISH traces and retrieve the pulse field E(x,y,t) or equivalently E(x,y,ω), to generate movies revealing the field structure induced by propagating modes, due to their differences in field spatial distribution, modal propagation velocity and modal dispersion inside the fiber. We launch femtosecond pulses near 800nm from Ti: Sapphire laser to investigate linearly polarized modes LP01, LP11, LP02 and LP21 in multimode fibers.
We demonstrate a simple single-shot device, called Spatially and Temporally Resolved Intensity and Phase Evaluation Device: Full Information from a Single Hologram (STRIPED FISH), for completely characterizing the intensity and phase of an arbitrary ultrashort pulse in space and time (x,y,t). Improvements are made on the measurable bandwidth, aberrations eliminations, and the intensity uniformity of the multiple holograms in our device. To demonstrate the capability, we perform single-camera-frame measurements of spatiotemporally complex subpicosecond crossed and chirped double pulses from a Ti:Sapphire oscillator. To display the resulting four-dimensional intensity-and-phase data, we generate intuitive movies of the measured pulses based on our newly-developed method.
We study multi-shot intensity-and-phase measurements of unstable trains of ultrashort pulses using two-dimensional spectral shearing interferometry (2DSI)  and self-referenced spectral interferometry (SRSI)  in order to identify warning signs of pulse-shape instability in these methods. 2DSI can signal instability with reduced fringe visibility, although this effect is very small when using small shears appropriate for large temporal support. SRSI can reliably indicate instability when two measured spectra are compared to an independent spectrum and a retrieved reference spectrum.
We study multi-shot intensity-and-phase measurements of unstable trains of ultrashort pulses using spectral-phase interferometry for direct electric-field reconstruction (SPIDER), second harmonic generation (SHG) frequency-resolved optical gating (FROG), polarization gate (PG) FROG, and cross-correlation FROG (XFROG). An analytical calculation suggests that SPIDER cannot indicate instability in pulse trains well. Simulations confirm this and demonstrate that SPIDER only measures the coherent artifact. Further, the presence of instability cannot be distinguised from benign misalignment effects in SPIDER. FROG methods suggest instability by exhibiting clear disagreement between measured and retrieved traces.